Among projection-type display devices (such as projectors) that project images onto a screen, projection-type display devices are known that use, as the light source, LEDs in place of high-pressure mercury lamps.
FIGS. 1A and 1B show examples of light source drive unit 200 that lights an LED. Light source drive units 200 shown in FIGS. 1A and 1B are known as constant-current step-down DC-DC converters.
In FIGS. 1A and 1B, voltage controller 104 controls the ON/OFF timing of switching elements 105 and 106 such that the voltage between the terminals of voltage-detecting resistor 102 that is connected in a series with LED 101 is equal to the reference voltage that is generated in reference voltage generation unit 103.
More specifically, in voltage controller 104, error amplifier 104a detects the difference between the reference voltage and the voltage between the terminals of voltage-detecting resistor 102. PWM unit 104b generates a PWM signal according to the difference detected by error amplifier 104a. Driver 104c controls the ON/OFF state of switching elements 105 and 106 according to the PWM signal generated by PWM unit 104b. Driver 104c turns switching element 106 OFF when switching element 105 is turned ON, and turns switching element 106 ON when switching element 105 is turned OFF.
Inductor 107 supplies current to LED 101 by means of electromagnetic induction that is generated according to the ON/OFF state of switching elements 105 and 106.
In FIG. 1A, the cathode side of LED 101 is connected to GND (ground), whereby voltage of a positive value is supplied from AC/DC voltage converter 108. In FIG. 1B, on the other hand, the anode side of LED 101 is connected to GND, whereby voltage of a negative value is supplied from AC/DC voltage converter 108.
In Patent Document 1, a projection-type display device is disclosed in which LEDs are used as the light source and, moreover, in which the lighting time of the LEDs in unit time is regulated according to the luminance indicated by image signals.
In the projection-type display device described in Patent Document 1, LED lighting duty cycle that indicates the proportion of the lighting time of the LED with respect to the unit time changes dynamically according to the luminance indicated by an image signal.
The function by which the LED lighting duty cycle is controlled according to the luminance indicated by image signals is hereinbelow referred to as “dynamic dimming capability.”
FIGS. 2A and 2B are figures for describing an example of the dynamic dimming capability, and more specifically, for showing the relation between the luminance (image signal level) indicated by an image signal and the LED lighting duty cycle. In FIG. 2A, graph “a” shows the transition of the image signal level, and in FIG. 2B, graph “b” shows the transition of the LED lighting duty cycle.
The technology for controlling the LED lighting duty cycle is realized by implementing PWM (pulse-width modulation) control of the drive signal (for example, a drive current signal) that is supplied to an LED.
The frequency of the LED drive signal that is implemented by PWM control is typically set within a range (200 Hz-1 kHz) that an LED can follow and does not recognize fluctuation of the brightness.
The frequency range (200 Hz-1 kHz) that is used as the frequency of the drive signal is contained within the range of frequencies (20 Hz-20 kHz) that the human ear can hear.
As a result, when a projection-type display device is equipped with a light source drive unit having an inductor such as shown in FIG. 1A or FIG. 1B, the problem arises that the inductor within the light source drive unit produces a noise (squeal) depending on the frequency of the drive signal.
However, most projection-type display devices are equipped with a cooling fan for cooling the components within the projection-type display device, and the noise of the inductor is drowned out by the drive sound of this cooling fan.